1. Field of the Invention
This invention relates to an improved shot peening machine and more particularly to such a shot peening machine in which there is provided a minimum number of components within a critical blast region thereof which components are subjected to wear and must be periodically replaced.
2. Description of the Prior Art
It is well known that the fatigue life of springs or similar types of work pieces that are subjected to repeated flexing stresses may be increased by cold working of the fibers at and near the surface of the metal by shot peening or blasting. For larger coil springs such as those used in automobile suspensions, it is not uncommon to provide a shot peening machine for conveying the springs through a fan-shaped shot stream with the springs being rotated about their axis for substantially uniform cold working of the entire surface.
A number of such shot peening machines have been heretofore successfully employed and include blast wheels and shot recirculation, fines separation and dust collecting systems which are well known to those skilled in the art. Although shown in a simplified form, U.S. Pat. No. 2,249,677 includes such a machine and, in one embodiment thereof, discloses a pair of spaced parallel spinner rolls which are mounted for rotation to support and rotate each spring being conveyed through the machine. Specifically, the springs are conveyed by a belt and finger arrangement which is located between the spinner rolls with each finger being capable of advancing an individual spring therethrough. A similar machine is shown in a simplified form in U. S. Pat. No. 2,341,674.
Still another prior art shot peening machine is shown in U.S. Pat. No. 3,383,803, which includes some discussion and explanation of the blast wheels and the shot recirculation system for returning shot to the blast wheels. The blast wheels provide the shot to an enclosed blast cabinet which is intended to contain all of the shot being used during the process. The prior art machine disclosed therein, however, utilizes a different means than discussed hereinabove for conveying and rotating a spring therethrough. This method is limited to coil springs which are formed with non-flattened ends. An elongated rotatable support roll extends continuously lengthwise through the machine and includes a plurality of annular ribs extending radially outwardly from the periphery of the roll. The ribs are axially spaced one from the other by a distance corresponding to the coil pitch of each coil spring to be supported thereon. Additionally, a pair of guide rollers are mounted for rotational movement about an axis parallel with the support roll but are spaced laterally one from the other and above the support roll. The space defined by the rolls is intended to loosely receive each coil spring therein. The guide rolls also extend continuously lengthwise through the machine and corresponding rotation of the guide rolls and the support roll cause rotation of the coil spring which results in it being conveyed through the machine as each coil spring is progressively advanced along the annular ribs of the support roll.
Still another prior art shot peening machine is disclosed in U.S. Pat. No. 3,604,158. This machine is also intended to rotate and convey cylindrical work pieces such as coil springs but utilizes a different conveying method than those described hereinabove. In order to provide for the transportation of each spring through the blast cabinet, a feed roll having a screw flight is mounted for rotation to progressively convey the springs through the machine. To maintain the springs in a spaced-apart relationship to insure that the shot can be directed to the ends thereof, the pitch of the screw is greater than the overall length of a spring. The spring is maintained in place on the feed roll by a pair of guide rolls which are mounted in a common horizontal plane slightly above the feed roll at a distance which is less than the spring diameter. Throughout the peening process, the guide rolls and feed roll are caused to rotate to impart a rotating motion to the springs to insure that the entire surface of each will be properly cold worked. In another embodiment disclosed in U.S. Pat. No. 3,604,158, a pair of fixed or rotating support rolls support a coil spring therebetween but axial movement of each spring is produced in a manner similar to those disclosed in the prior art devices mentioned hereinabove. Specifically, there is provided a plurality of fingers on a moving conveyor chain which is mounted between and below the support rolls. This embodiment still employs rotating guide rolls above and at opposite sides of the springs to produce rotation thereof as each spring is axially moved through the blast cabinet.
While each of the above described prior art shot peening machines appears to satisfactorily transport and rotate springs through a blast cabinet to insure proper shot peening of the entire surface of such springs, each of these machines includes features which cause them to be expensive to provide and costly to maintain. It has been found that any element located inside the blast cabinet in a critical blast region (a region in which shot which is provided by the blast wheels will still be traveling near to its maximum impact speed) is subjected to extensive wear. To increase the life of these elements, they are often manufactured of relatively expensive wear-resistant material, such as manganese steel alloy. Even though extensive manganese steel alloy protection plates and deflectors made of stock material may be provided to reduce and minimize this life-shortening damage to the various, more expensive, machined and manufactured components within the blast cabinet, they must nevertheless be periodically replaced and are just as expensive to replace as they were when initially provided.
Additionally, although the prior art shot peening machines discussed hereinabove appear to be primarily concerned with conveying and rotating coil springs through the critical blast regions of their respective shot peening machines, the particular components used for these purposes are also used to simultaneously feed the springs into the critical blast region and discharge the spring from the critical blast region. Accordingly, the various components required for the primary functions in the critical blast region must by necessity be sufficiently long to perform these additional functions. Any design requirement that would add to their overall length would tend to increase their initial cost and any future costs periodically required for their replacement.